A security document including a sheet like substrate having a transparent area or “window” formed from a portion of transparent plastics material has previously been proposed. The window formed in the security document is particularly suited for incorporating a security device such as an embossed image or an optically variable device, such as a diffraction grating or hologram.
Security devices are required to attest to the authenticity of the security document. The devices are required to prevent counterfeiting of security documents and in the case of overt security devices, should be easily and quickly identifiable on the document. A further requirement for security documents is that they are cost effective to produce and are durable enough to withstand everyday use, such as the use encountered by banknotes.
WO 2006/133512 discloses a security document or device having colour shifting inks. An ink is provided which provides at least one at least one colour shift between a first colour and a second distinct colour depending upon the viewing angle.